In the future, the most common transfer mode for the broadband ISDN will be the ATM (Asynchronous Transfer Mode). ATM is a statistical multiplexing and switching method which is based on fast packet switching concept allowing an asynchronous operation between the sender clock and the receiver clock. The difference between the clocks is solved by providing temporary storage for input packages and by inserting or removing packets not containing useful information, i.e. emptying of assigned packets of data, in the information stream. ATM provides dedicated circuits for voice, data and video communications by dividing the information flow within each of these three types of traffic into individual traffic "cells", each being a kind of "package" including a header comprising an address or directions specifying the location to which the information carried within the cell should be delivered, and data about the information. Direction instructions are added to the information carried by the cell in the form of a label, which is processed by an ATM switch as the cell is routed through it.
A network having this kind of transfer mode is capable of transferring all sorts of services, for example low speed transfer, such as telemetry, telecontrol low speed data etc, medium speed transfer, such as hifi sound, video telephony etc, and high speed transfer, such as high quality video distribution or the like. Traffic cells are sent in a continuous data stream; traffic cells not containing useful information are inserted and/or removed in the datastream when necessary. No handshaking operations are provided in the network; instead the network has a low BER (Bit Error Rate). Nowadays, the concepts for telecommunications allow large Gigabit ATM switches to be designed based on a strictly non-blocking matrix. The matrix has then been simply increased to the desired size, by interconnecting switchcore chips. A 40 Gbps switch can most likely be made by using the proven technology of today.
However, as the volume of communication traffic increases, and particularly the demand for interaction between a customer and for instance the public data net INTERNET, demand has arisen for switches able to manage even higher rates.
According to the ATM principle a number of incoming links transport the ATM information to the ATM switch where depending on the value of the header of the incoming traffic cell the information is switched to an outgoing link. The incoming header and the incoming link number of the traffic cell are used to access a translation table in a switchcore. The result of the access to the translation table is an outgoing link and a new header value for the cell.
In a prior multiport controlled access ATM switch described in the U.S. Pat. No. 5,467,347 a multiport device included one switchcore circuit to which the ports were connected. This switchcore circuit was able to serve a number of A external links, where A for example is 16, up to a rate of 40 Gbps. There could be different inports and outports, or each port could be used as a combined input/output port.
However, nowadays there is a demand to be able to serve up to 128 links for rates up to and above 80 Gbps.
IBM Technical Disclosure Bulletin, Vol. 36, No. 11, November, 1993, (Armonk, N.Y., USA), "First-in First-out Queuing in Single Stage Switching Networks Built from Multiple Identical Packet Switching Modules", p. 517-520, discloses briefly on page 517 a modular switch structure, in which the modules can be constructed of small modules and when needed be combined to a matrix of four modules. At paralleled outputs there is provided a control logic which ensures that only one port is active at the time.